The catabolic state of the body consuming itself is characteristic of fasting and onset of infection. Detection of changes in the ratio of 13C to 12C in exhaled carbon dioxide has shown promise in preliminary evaluations for very early detection of the onset of a catatabolic state. However, progress in studying and utilizing this method has been severely limited due to the lack of a suitable real-time carbon dioxide isotope ratio analyzer. Picarro, Inc. proposes to develop a low-cost, real-time carbon dioxide isotope ratio analyzer based on cavity ringdown spectroscopy. During phase I, Picarro will design, build, and test an instrument designed to measure changes in the carbon dioxide isotopic ratio of exhaled breath to better than 0.2 parts per mil in less than 15 minutes. Picarro will provide this instrument to Dr. Warren Porter at the University of Wisconsin, Madison who will utilize the instrument to measure the carbon istope ratio changes in breath from rats infected with an endotoxin. It is anticipated that the isotopic ratio will track the onset and recovery from this infection. The real-time monitoring of exhaled carbon isotope ratios will enable new research in early disease diagnosis and nutritional monitoring. [unreadable] [unreadable] [unreadable]